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<h2><span class="refentrytitle">Function template check</span></h2>
<p>boost::xpressive::check — For adding user-defined assertions to your regular expressions. </p>
</div>
<h2 class="refsynopsisdiv-title">Synopsis</h2>
<div class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../xpressive/reference.html#doxygen.regex__actions_8hpp" title="Header &lt;boost/xpressive/regex_actions.hpp&gt;">boost/xpressive/regex_actions.hpp</a>&gt;

</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> T<span class="special">&gt;</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span> t<span class="special">)</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.44.5.4.48.4"></a><h2>Description</h2>
<p>
A  <a class="link" href="../xpressive/user_s_guide.html#boost_xpressive.user_s_guide.semantic_actions_and_user_defined_assertions.user_defined_assertions">user-defined assertion</a> is a kind of semantic action that evaluates a Boolean lambda and, if it evaluates to false, causes the match to fail at that location in the string. This will cause backtracking, so the match may ultimately succeed.</p>
<p>To use <code class="computeroutput"><a class="link" href="namespaceboost_1_1xpressive_1a98dd0902c3d29a502836077cfa6e0130.html" title="Function template check">check()</a></code> to specify a user-defined assertion in a regex, use the following syntax: </p>
<pre class="programlisting"><a class="link" href="structboost_1_1xpressive_1_1basic__regex.html" title="Struct template basic_regex"><span class="identifier">sregex</span></a> <span class="identifier">s</span> <span class="special">=</span> <span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1afae3ed7c3a8ebd3c2f06a943ff974a58.html" title="Global _d"><span class="identifier">_d</span></a> <span class="special">&gt;&gt;</span> <a class="link" href="namespaceboost_1_1xpressive_1afae3ed7c3a8ebd3c2f06a943ff974a58.html" title="Global _d"><span class="identifier">_d</span></a><span class="special">)</span><span class="special">[</span><a class="link" href="namespaceboost_1_1xpressive_1a98dd0902c3d29a502836077cfa6e0130.html" title="Function template check"><span class="identifier">check</span></a><span class="special">(</span> <span class="identifier">XXX</span> <span class="special">)</span><span class="special">]</span><span class="special">;</span> <span class="comment">// XXX is a custom assertion</span>
</pre>
<p> The assertion is evaluated with a <code class="computeroutput"><a class="link" href="structboost_1_1xpressive_1_1sub__match.html" title="Struct template sub_match">sub_match</a>&lt;&gt;</code> object that delineates what part of the string matched the sub-expression to which the assertion was attached.</p>
<p><code class="computeroutput"><a class="link" href="namespaceboost_1_1xpressive_1a98dd0902c3d29a502836077cfa6e0130.html" title="Function template check">check()</a></code> can be used with an ordinary predicate that takes a <code class="computeroutput"><a class="link" href="structboost_1_1xpressive_1_1sub__match.html" title="Struct template sub_match">sub_match</a>&lt;&gt;</code> object as follows: </p>
<pre class="programlisting"><span class="comment">// A predicate that is true IFF a sub-match is</span>
<span class="comment">// either 3 or 6 characters long.</span>
<span class="keyword">struct</span> <span class="identifier">three_or_six</span>
<span class="special">{</span>
    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><a class="link" href="structboost_1_1xpressive_1_1sub__match.html" title="Struct template sub_match"><span class="identifier">ssub_match</span></a> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">sub</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">sub</span><span class="special">.</span><a class="link" href="structboost_1_1xpressive_1_1sub__match.html#doxygen.structboost_1_1xpressive_1_1sub__match_1ac906824245509413049ff3b29471ca01"><span class="identifier">length</span></a><span class="special">(</span><span class="special">)</span> <span class="special">==</span> <span class="number">3</span> <span class="special">||</span> <span class="identifier">sub</span><span class="special">.</span><a class="link" href="structboost_1_1xpressive_1_1sub__match.html#doxygen.structboost_1_1xpressive_1_1sub__match_1ac906824245509413049ff3b29471ca01"><span class="identifier">length</span></a><span class="special">(</span><span class="special">)</span> <span class="special">==</span> <span class="number">6</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span><span class="special">;</span>

<span class="comment">// match words of 3 characters or 6 characters.</span>
<a class="link" href="../xpressive/reference.html#doxygen.namespaceboost_1_1xpressive_1ad82b5700d5ba5eb195cfce6d4007f0b4"><span class="identifier">sregex</span></a> <span class="identifier">rx</span> <span class="special">=</span> <span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1a3b217185880e704131c77830f43af5cb.html" title="Global bow"><span class="identifier">bow</span></a> <span class="special">&gt;&gt;</span> <span class="special">+</span><a class="link" href="namespaceboost_1_1xpressive_1a6d4422b6535a447d3eb0689827b0b8fe.html" title="Global _w"><span class="identifier">_w</span></a> <span class="special">&gt;&gt;</span> <a class="link" href="namespaceboost_1_1xpressive_1a88a7a6bc37a07db67a44936c6e0a3313.html" title="Global eow"><span class="identifier">eow</span></a><span class="special">)</span><span class="special">[</span> <a class="link" href="namespaceboost_1_1xpressive_1a98dd0902c3d29a502836077cfa6e0130.html" title="Function template check"><span class="identifier">check</span></a><span class="special">(</span><span class="identifier">three_or_six</span><span class="special">(</span><span class="special">)</span><span class="special">)</span> <span class="special">]</span> <span class="special">;</span>
</pre>
<p> Alternately, <code class="computeroutput"><a class="link" href="namespaceboost_1_1xpressive_1a98dd0902c3d29a502836077cfa6e0130.html" title="Function template check">check()</a></code> can be used to define inline custom assertions with the same syntax as is used to define semantic actions. The following code is equivalent to above: </p>
<pre class="programlisting"><span class="comment">// match words of 3 characters or 6 characters.</span>
<a class="link" href="structboost_1_1xpressive_1_1basic__regex.html" title="Struct template basic_regex"><span class="identifier">sregex</span></a> <span class="identifier">rx</span> <span class="special">=</span> <span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1a3b217185880e704131c77830f43af5cb.html" title="Global bow"><span class="identifier">bow</span></a> <span class="special">&gt;&gt;</span> <span class="special">+</span><a class="link" href="namespaceboost_1_1xpressive_1a6d4422b6535a447d3eb0689827b0b8fe.html" title="Global _w"><span class="identifier">_w</span></a> <span class="special">&gt;&gt;</span> <a class="link" href="namespaceboost_1_1xpressive_1a88a7a6bc37a07db67a44936c6e0a3313.html" title="Global eow"><span class="identifier">eow</span></a><span class="special">)</span><span class="special">[</span> <a class="link" href="namespaceboost_1_1xpressive_1a98dd0902c3d29a502836077cfa6e0130.html" title="Function template check"><span class="identifier">check</span></a><span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1ac007382aa804f2950e4b9d5087e11cc1.html" title="Global length"><span class="identifier">length</span></a><span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1a12ce0e9a6350027c3c11d11e54411e23.html" title="Global _"><span class="identifier">_</span></a><span class="special">)</span><span class="special">==</span><span class="number">3</span> <span class="special">||</span> <a class="link" href="namespaceboost_1_1xpressive_1ac007382aa804f2950e4b9d5087e11cc1.html" title="Global length"><span class="identifier">length</span></a><span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1a12ce0e9a6350027c3c11d11e54411e23.html" title="Global _"><span class="identifier">_</span></a><span class="special">)</span><span class="special">==</span><span class="number">6</span><span class="special">)</span> <span class="special">]</span> <span class="special">;</span>
</pre>
<p> Within a custom assertion, <code class="computeroutput">_</code> is a placeholder for the <code class="computeroutput"><a class="link" href="structboost_1_1xpressive_1_1sub__match.html" title="Struct template sub_match">sub_match</a>&lt;&gt;</code> That delineates the part of the string matched by the sub-expression to which the custom assertion was attached. </p>
<div class="variablelist"><table border="0" class="variablelist compact">
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<td><p><span class="term">Parameters:</span></p></td>
<td><div class="variablelist"><table border="0" class="variablelist compact">
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<td><p><span class="term"><code class="computeroutput">t</code></span></p></td>
<td><p>The UnaryPredicate object or Boolean semantic action.</p></td>
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<div class="copyright-footer">Copyright © 2007 Eric Niebler<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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